1. Field of Invention
The invention relates to a power supply technology, more particular to a power supply control circuit with frequency expansion modulation, a light emitting diode (hereinafter referring to as LED) driver and a display module using the same.
2. Related Art
As the development of the photo electronics and semiconductor technology, the flat panel display is also rising and flourishing. Among many flat panel displays, the liquid crystal display (hereinafter referring to as LCD) becomes the main stream in the display market due to the characteristics of high space utility efficiency, low power consumption, low radiation and low electromagnetic interference (EMI). The LCD comprises a liquid crystal display panel and a backlight module. Since the LCD panel cannot radiate light itself, a backlight module is required to provide a surface light source to the LCD panel to enable its display function. The conventional backlight module uses a cold cathode fluorescent lamp (CCFL) as the light source. However, as the development of the small display device and the power consumption demand of the small mobile devices, the LED is adopted for the light source of the backlight module.
FIG. 1 is a circuit diagram showing the conventional LED driver circuit. Referring to FIG. 1, the basic operation of the circuit is to utilize a boost converter to raise the input voltage to a conductible voltage level to drive a series of LEDs, and then to utilize a constant current controller to control a constant current flowing through the LEDs, so that the LEDs can be used to generate a stable light source.
In addition, in the above-mentioned example, the boost converter in FIG. 1 further comprises an inductor L101, a rectify diode D101, a capacitor C101 and a power control circuit IC101, wherein the power control circuit IC101 comprises a constant current control circuit CI, a feedback selection circuit FBS, a reference voltage generator REFG, an error amplifier ERRAMP, a pulse width modulation comparator PWMCMP, a saw-tooth generator STG, a mixer MIX, a clock generator CKG, a current sensing amplifier CSAMP, a set-reset latch SR-LATCH, a gate driver GD, a power transistor PM and a current sensing resistor RCS.
The operation of the boost converter is to convert the difference between the feedback voltage VFB and the reference voltage VREF to a current by the error amplifier ERRAMP and to charge the first order low-pass filter, which is composed of an external resistor Rc and an external capacitor Cc, to obtain an error voltage VEER. At this moment, a clock signal CK, which is generated by the saw-tooth ST, is utilized to set the Q output terminal of the set-reset latch SR-LATCH to a high voltage level and the high voltage level of the Q output terminal conducts the power transistor PM through the gate driver GD and the current IL starts to charge the inductor L101. Thereafter, the voltage on the current sensing resistor RCS is amplified by the current sensing amplifier CSAMP and mixed with the saw-tooth ST generated by the saw-tooth generator STG in the mixer MIX. Thereafter, a mixed saw-tooth signal MST is produced. When the voltage level of the mixed saw-tooth signal MST goes up to reach the error voltage VERR level, the pulse width modulation comparator PWMCMP outputs a high voltage after the comparison, and the high voltage is used for setting the Q output terminal of set-reset latch SR-LATCH to a low voltage level. The low voltage level of the Q output terminal turns the power transistor PM off through the gate driver GD and at the current IL stops charging the inductor IL101, due to the continuity of the inductor current on the inductor L101, the energy stored in the inductor IL101 is discharged at the output terminal VOUT through the rectified diode D101 and to drive load, and an operation cycle of the boost converter is complete.
Under the operation cycle of charging and discharging mentioned above, the pulse width modulation comparator PWMCMP generates a square wave with constant frequency, whose pulse width varies with the load, and the described above herein is the operating principle of the pulse width modulation power converter.
In today's world, the science and technology are changing day by day, many kinds of electrical products, communication products, and information products have become our necessities in our daily life. The usage of all these products suffers the non-continuity of current and voltage to result in an electric field and magnetic field, so that an interference phenomenon is formed. Such interference is an invisible pollution of our environment. The cell phone, television, telephone will not only be affected to reduce our life quality, but the safety of the flight is endangered, and even more the human body may be influenced. In view of this, more and more countries request that the electrical products must pass the Electromagnetic Compatibility (EMC) test before the products enter the market. No doubt the switch mode power supply (SMPS) is a major EMI source.
The fundamental frequency and its harmonics are the main source of the electromagnetic interference of the common used pulse width modulation (PWM) power converter. However, the solutions of many manufacturers to against the electromagnetic interference are still stayed on building electromagnetic shields to block the interference source, and there is still no effective solution to solve EMI issue from its source.